A tempered martensitic heat resisting steel has, as represented by ASME T91, P92, P122, excellent high temperature creep strength, and is used in heat resistance and pressure resistant components of a high temperature plant typically including a thermal power plant and atomic power plant. In many cases, however, pressure resistant components and pressure resistant parts of a tempered martensitic heat resisting steel in a high temperature plant are manufactured by welding, and a weldment has a different structure from that of the base metal, consequently, its creep strength becomes lower than that of the base metal. Therefore, the creep strength of a weldment part is an important factor for the performance of a high temperature plant.
The welding procedure used for heat and pressure resistant components in a high temperature plant includes TIG welding, shielded metal arc welding, submerged arc welding and the like, however, in any method, zone changing microstructure by applied heat during welding (heat affected zone, HAZ) are generated in a weldment. HAZ of a tempered martensitic heat resisting steel shows change in microstructure by exposure to temperatures of AC1 point or higher, even if temperature momentarily increases during welding, therefore, there is a problem of decrease in creep strength as compared with a base metal (none heat affected zone). That is, when a creep test is conducted using a welded joint containing a base metal and a weldment as a specimen parallel part, rupture occurs in HAZ.
When a tempered martensitic heat resisting steel is exposed to temperatures of AC1 point or higher, ferrite as a base phase of a tempered martensite structure is transformed into austenite. The microstructure of austenite newly generated in this transformation is formed so as to break the microstructure of original tempered martensite. That is, austenite grains generated at temperatures of AC1 point or higher nucleate and grow so as to erode the microstructure of ferrite grains, independent of the microstructure of ferrite grains as a base phase of tempered martensite. At temperatures of AC3 point or higher, the base phase is utterly transformed to austenite, and the microstructure of original tempered martensite is lost.
Therefore, at temperatures around AC1 point to AC3 point, austenite grains are newly formed in large amount, as a result, a microstructure with very fine grain size (fine-grained HAZ) is formed. At temperatures around AC3 point or higher to melting temperature, austenite grains become coarse, and a microstructure having relatively larger prior austenite grain size (coarse-grained HAZ) as compared with the microstructure of portions exposed to temperatures around AC1 point to AC3 point.
In commercially available P92, P122 and the like, the prior austenite grain size in a base metal is larger than the prior austenite grain size of a coarse-grained HAZ. That is, in HAZ of P92, P122 and the like normalized at 1090° C. or lower, prior austenite grain size is finer than that of a base metal. As a result to date of investigation of the creep strength of a welded joint of a tempered martensitic heat resisting steel such as P92, P122 and the like, it is known that creep strength decreases remarkably at a fin-grained HAZ. In the case of a welded joint of a tempered martensitic heat resisting steel such as P92, P122 and the like, TYPE-IV fracture at a fine-grained HAZ occurs, and at 650° C., the creep rupture time decreases to about 20% of a base metal.
For suppression of deterioration in creep strength at a fine-grained HAZ, production of Ti, Zr, Hf carbonitride in a base metal is proposed (see, e.g. patent document 1). It is also proposed that one or more kinds of Mg-containing oxide grains having a grain size of 0.002 to 0.1 μm and composite grains having a grain size of 0.005 to 2 μm composed of a Mg-containing oxide and a carbonitride precipitated using the oxide as a nucleus are contained in a total amount of 1×104 to 1×108/mm2 (see, e.g. patent document 2). Further, suppression of deterioration in the creep strength of HAZ by a Ta oxide is proposed (see, e.g. patent document 3). Furthermore, there are proposals such as suppression of deterioration in the creep strength of HAZ by optimization of balance of W and Mo, or by addition of W and by a carbonitride of Nb, Ta (see, e.g. patent documents 4, 5). In addition, suppression of deterioration in the creep strength of HAZ according to solid-solution strengthening of HAZ and improvement in ductility of HAZ by addition of Cu and Ni is proposed (see, e.g. patent document 6).
However, in a creep test of a welded joint of P92, P122 and the like, fracture observed in HAZ, particularly in a fine-grained HAZ is caused by linkage of voids formed at grain boundaries mainly at prior austenite grain boundaries. In view of such fracture mechanism, small size of prior austenite grain is believed to be one of important factors for deterioration in the creep strength of HAZ since small prior austenite grain size increases the number of void nucleation sites and linkage of voids easily occurs.
The present invention has been made in view of the circumstances as described above, and an object of the present invention is to provide a welded joint of a tempered martensitic heat resisting steel in which formation of fine-grained HAZ causing remarkable decrease in creep strength is suppressed.
Patent document 1: Japanese Patent Application Laid-Open (JP-A) No. 08-85848
Patent document 2: JP-A No. 2001-1927761
Patent document 3: JP-A No. 06-65689
Patent document 4: JP-A No. 11-106860
Patent document 5: JP-A No. 09-71845
Patent document 6: JP-A No. 05-43986